Industrial quality open-path laser-based trace gas monitors and alarms have been commercially available since their introduction in the 1990s. These devices are now produced, sold, installed, maintained, and serviced by several companies worldwide. They are deployed for monitoring routine industrial and agricultural emissions or leaks of hazardous gases such as hydrogen fluoride, hydrogen sulfide, and ammonia. They are used in environmental research and monitoring applications for greenhouse gas and pollution detection and measurement.
Most current laser-based open-path sensors utilize a mono-static configuration: a laser beam projects from a transceiver to a distant retroreflector, wherefrom the laser beam returns to the transceiver. The return beam carries information describing the amount of target gas within the beam path. See U.S. Pat. No. 7,075,653 to Rutherford, the entire disclosure of which is hereby incorporated by reference.
While current commercially-available open-path gas monitors are capable of detecting large gas leaks, they suffer from limited sensitivity to small leaks (e.g. leaks smaller than the natural variability of the background gas concentration), or intermittent leaks. Furthermore, stability of alignment between the transceiver and retroreflector, as well as obstacles or obscurants in the laser path, can cause measurement noise and drift or dropout that limit leak detection ability.